Field of the Invention
The present invention has its application within the telecommunications sector and, in particular, in medium access control protocols in a wireless machine-to-machine communication network.
Related Art
Machine-to-machine (M2M) communication networks provide remote management of a plurality of distributed nodes with sensing and/or actuating capabilities, and enable to obtain data remotely from said nodes or control devices connected to said nodes. The nodes are controlled by one or more gateways, the gateways being also responsible of communications with other elements of the network (such as servers or applications), and with external networks. Communication between gateway and nodes can be either wired or wireless, in the latter case typically operating in the Industrial, Scientific and Medical (ISM) bands. Wireless M2M communication networks can be applied to a variety of fields, including smart metering (electricity, water and gas utilities), remote sensing of physical constants (river water levels, bridges safety, etc.), radiofrequency identification and localization of objects, and remote actuating over physical devices (street lights, traffic lights), among others.
In a typical wireless M2M network, a very large number of nodes need to be supported, each of them producing small amounts of traffic. Typically, uplink traffic (traffic from the node to the gateway) is generated simultaneously at a plurality of nodes as a result of a request from the gateway. As a consequence, step-like traffic may be generated (that is, a large sudden increase in the traffic load of the network), when a large amount of nodes try to access the medium simultaneously, even if each of them is trying to send a small amount of data. In addition to as a response to requests from the gateway (pull communications), traffic can be generated spontaneously at the nodes (push communications), for example as a result of an alarm triggered by a sensor. In this case, communication delays are particularly critical, and the information from the node needs to be sent without waiting for a periodical survey from the gateway. Furthermore, a particular event may generate an alarm simultaneously in multiple nodes, causing them to try to access the medium at the same time. This scenario also results in a step-like traffic that needs to be properly handled by the network.
Therefore, M2M networks require MAC (Medium Access Control) protocols that are capable of efficiently handling both a low load communication scenario in which traffic is generated randomly at the nodes, and a situation in which a very large number of nodes try to access the medium simultaneously as per gateway request. This is challenging for conventional MAC protocols as contention-based protocols cannot handle heavy traffic, whereas deterministic protocols require a priori knowledge of the application QoS (Quality of Service) requirements (bandwidth, delay, etc.), as they incur in heavy energy and network efficiency penalties when they need to adapt to changing conditions.
Some additional problems inherent to wireless M2M networks need to be addressed by the MAC protocol. First, the number of nodes may be unknown a priori, and may vary during the operation of the M2M network, as additional nodes are deployed or some of the nodes are removed. The MAC protocol must hence be able to synchronize the new nodes and allow them to receive and transmit data in a deterministic and efficient way, i.e. without packet collisions. Second, ISM bands do not require a license, which may cause interference from adjacent networks operating in the same frequency band. Thus, the MAC protocol must be able to withstand this interference and reduce its impact in ongoing communications. Third, nodes in a wireless M2M network typically operate using batteries, which have to last for long periods of time as they cannot be changed frequently due to the cost overhead it would represent. Therefore, it is of critical importance that the MAC protocol minimizes energy consumptions derived from data transmission and reception.
All the above particularities of wireless M2M networks need to be addressed when designing an appropriate MAC protocol, and in particular, when designing network synchronization, data transmission, and robustness against interferences.
Regarding network synchronization, two main alternatives are known: preamble sampling and time synchronization. In systems based on preamble sampling, nodes stay by default in a sleep state (radio communication means turned off), periodically turning on the radio to sample the communication channel for a short period of time. During said short periods, the nodes measure the energy level present in the communication channel and determine if a packet is being transmitted. In case energy is present in the channel and a packet is successfully received, it is used to synchronize the node, which either keeps the radio on to transmit or receive a packet right after or goes back to the sleep state and schedules to wake up in the future (for example, after a certain period of time determined by the packet contents). Different preamble sampling mechanisms are known. For example, this concept can be implemented by modulating a carrier, enabling the receiving node to detect that the signal is present in the channel and to decode said signal. In another example, the gateway sends a plurality of short packets, so the node can detect that a signal is being transmitted over the channel, and is able to correctly receive the packet.
Using the preamble sampling approach in a M2M context, whenever the gateway has some data to receive from the end nodes, it sends a train of small packets (wake-up packets) for, at least, the same length of the end nodes sampling time, therefore ensuring that all nodes will receive at least one of the packets and act accordingly, i.e. keep the radio on to receive a packet. Preamble sampling approaches avoid maintaining a permanent synchronization between gateway and nodes, which is interesting for networks that need to support high node mobility. This approach can be used in both uplink and downlink communications, thus reducing energy consumption both at the gateway and the nodes. However, by periodically turning on the radio, a considerable amount of energy is still consumed. Furthermore, if the nodes are in sleep state for long periods, energy consumption at said nodes is reduced, but as a trade-off, the gateway needs to send more wake-up packets, hence increasing its energy consumption and reducing the data capacity of the channel. It should also be noted that in scenarios with high traffic load, the preamble sampling process needs to be constantly performed, increasing energy consumption.
Alternatively, time synchronization is based on a common master clock between the gateway and all the nodes. The master clock provides a common time reference for all the elements of the network, hence synchronizing all communications. Once all the elements are synchronized, the gateway distributes the network scheduling to all the nodes, which can therefore determine when to wake up for data transmission and/or reception of data packets. The main disadvantage of this configuration is that if burst traffic is expected (large amounts of traffic concentrated in short periods of time), it is necessary to reserve time slots that are seldom going to be used, therefore misusing network resources and increasing energy consumption. Furthermore, whenever a node joins or leaves the network, the schedule needs to be recalculated and redistributed, which implies a large energetic cost and a significant network efficiency reduction.
There are two types of time synchronization, namely hard and soft. Hard synchronization occurs when a node wants to join the network and needs to align its internal clock with the master clock. For this purpose, the gateway periodically transmits a control packet (beacon) comprising information of the master clock. In order to synchronize with the gateway, nodes turn on the radio and wait until a beacon from the gateway is received. Upon receiving the beacon the node clock is aligned to the gateway, allowing the node to communicate with the gateway. The main advantage of time synchronization over preamble sampling is that it reduces the energy overhead derived from periodically sampling the channel for on-going activity. However, if synchronization is lost the node needs to start the synchronization process. Thus, maintaining synchronization is a crucial aspect of time-synchronized networks. Soft synchronization (also named re-synchronization) is required to periodically to compensate the relative drifts between master and node clocks, i.e. construction imperfections and temperature changes, and avoid losing synchronization. Typically nodes periodically re-synchronize using the beacon transmitted by the gateway.
Network synchronization needs to be combined with a medium access technique that enables data transmission, which in traditional MAC protocols for M2M communications are either based on random or deterministic access. For example, a preamble sample system based on trains of waking-up packets can be used in combination with a random medium access, such as slotted Aloha, to enable data transmission for the nodes. Likewise, random access can also be applied to networks synchronized with a master clock transmitted by the gateway in the beacon packet.
In random access MAC protocols, whenever a node has a packet to transmit to the network, the node follows a predefined set of rules without central coordination from the gateway. Since nodes can access the shared medium at any random time, a collision between two packets may occur (that is, two nodes send data simultaneously using the same channel). In case of collision between two or more data packets, the data cannot be correctly decoded, and the data packets need to be transmitted again until no collision occurs.
The first random access MAC protocol was Aloha, in which nodes may try to transmit a packet at any time regardless of other stations. In low traffic scenarios, the chance of collision is low, but when traffic increases, so do collisions, greatly reducing network performance (network performance up to 18%). This mechanism was improved with slotted Aloha, in which nodes can only start transmitting at the beginning of time slots of fixed length. This reduces the chance of collision, but still provides poor performance in scenarios with high traffic (up to 36%).
To further increase network performance, Carrier Sense Multiple Access (CSMA) mechanisms were introduced, in which the nodes monitor the communication medium prior to packet transmission. If a node detects an ongoing transmission from another mode, it refrains from sending any additional transmission for a certain amount of time, thus avoiding the packet collision. According to different implementations, when a node detects an ongoing transmission, the node may either keep monitoring the medium until the end of the transmission (1-persistent CSMA), or only check the medium again after a certain time (non-persistent CSMA). 1-persistent CSMA improves network performance, whereas non-persistent CSMA reduces energy consumption. Nevertheless, collisions may still occur in CSMA systems. For example, two different nodes that are outside of each other's communication range, may transmit data to the same gateway. Since transmissions from the first node do not reach the second node, said second node may consider the medium is free and start transmitting, resulting in a collision and therefore requiring the data to be retransmitted.
This problem can be solved by using data communication requests (RTS, Request to Send), which are answered by permissions from the gateway (CTS, Clear to Send). Previous to transmitting a data packet, the node transmits a RTS in order to reserve the channel, and waits to receive a CTS from the gateway. Since nodes cannot transmit without receiving a CTS message, collisions between data packets are prevented, although collisions between RTS messages may still occur. Additionally, this system reduces network efficiency, especially in low-traffic scenarios, as a consequence of the time used for transmitting and receiving RTS/CTS packets.
In deterministic access MAC protocol, network resources are assigned to the nodes (either statically or dynamically), by means of time, frequency and/or code multiplexing. Static deterministic access is not suitable for wireless M2M communications because the network has finite resources that would be divided among a great number of nodes that would very seldom use them to transmit data, greatly reducing network efficiency. Dynamic deterministic access is also not suitable for wireless M2M networks, as the number of nodes is highly dynamic and would require to constantly reassign network resources, which implies recalculating and redistributing the network schedule each time a node joins or leaves the system. Additionally, in order to join the network and request resources, nodes need to be able to communicate with the gateway on an ad hoc basis, i.e. without any previous knowledge of the status of the communication network. This results in resources statically allocated for management traffic and, as a consequence, a reduction in network performance.
Finally, several techniques to increase communication robustness against interferences are known. This issue is particularly critical in wireless M2M communication networks, since no license is required to operate in the ISM bands. Therefore, nearby M2M networks can be operating in overlapping frequencies, hence hindering communications unless specific measures against this problem are taken. Some of the alternatives known in the state of the art are Direct Sequence Spread Spectrum (DSSS), Frequency Hopping Spread Spectrum (FHSS) and Channel Hopping (CH).
Direct Sequence Spread Spectrum is based on spreading a baseband signal into a larger bandwidth than strictly required to improve resistance against noise and interference. It also enables multiple users to share a single channel by using different codes in their communications. In order to obtain a DSSS signal, the data to be transmitted is multiplied by a pseudo-random signal, that is, a sequence of positive and negative values (1 and −1), at a frequency higher than that of the baseband signal.
Frequency Hopping Spread Spectrum is based on periodically changing a carrier that modulates the baseband signal. The carrier that modulates the baseband signal is selected using a pseudo-random sequence that is known by both the transmitter and the receiver. It should be noted that in a FHSS scheme, the rate at which the carrier changes is of the same order of magnitude as the baseband signal rate.
Finally, channel hopping is based in switching the channel used for the transmission of each packet, following a sequence known by the transmitter and the receiver. The main difference between with FHSS is the rate at which the carrier frequency changes. Whereas in FHSS the change can take place at a symbol level (each symbol is transmitted with a different carrier), in CH the channel is changed for each complete packet.
Recently, hybrid MAC protocols have been proposed for data transmission. These hybrid MAC protocols do not require a central entity to calculate and distribute network schedule, yet enable nodes to communicate with each other while minimizing the probability of packet collision. An example of an hybrid MAC protocol is Distributed Queuing (DQ), originally developed for distributing cable television signals, and later adapted to Wireless Cellular Systems (GSM) and Wireless Local Area Networks (WLAN). In DQ-based systems, data packet transmission is self-organized among nodes using two queues. The first queue, named Collision Resolution Queue (CRQ), enables nodes contending for packet transmission to resolve their status by means of applying a blocked Split Tree Algorithm (STA) in consecutive attempts. The second queue, named Data Transmission Queue (DTQ), enables nodes that have successfully resolved their collision by means of using the CRQ and the STA to transmit a data packet in the network without contention. U.S. Pat. Nos. 6,292,493 B1 and 6,408,009 B1 present two examples of hybrid medium access using Distributed Queuing. Nevertheless, these examples have not been optimized for wireless M2M networks and, therefore, do not deal with its particular problems, such as the use of a shared channel for uplink and downlink communications among a high and variable number of nodes. Furthermore, they do not provide robust communications against interferences nor are energetically efficient.
Taking into account all the aspects presented above, MAC protocols known in the state of the art fail to provide an efficient solution for medium access control in wireless M2M networks with a large and variable number of nodes with limited energy resources and which can operate both in pull and push communications. There is hence the need of a method and apparatus for data transmission and reception in such networks, which provides high network efficiency both in light-traffic and heavy-traffic scenarios, while preserving low power consumption at the nodes and communication robustness against interferences.